So is this it? Have we found alien life? Are we no longer alone? Is this, in short, the biggest story of all time?

Well, potentially. Sort of. No, not really. Not yet. Prof Vogt seems to have said this in a bit of a moment of understandable excitement, because he reels it back in a bit in his very next sentence: "I would say, my own personal feeling is that the chances of life on this planet are 100 percent. I have almost no doubt about it." (Italics mine.)

Almost no doubt is, of course, not the same as 100 per cent certainty. Prof Vogt clearly believes it is very likely that Gliese 581b is home to life, but it sounds as though, under the glare of the cameras, he may have overstated his case.

Dr Lewis Dartnell, a UCL astrobiologist who specialises in the possibility of microbial life on Mars, says that it's "undeniably very, very exciting", but it's far too early to say anything about life existing on it.

"We don't know how Earth-like it really is", he says. "It's a small rocky planet, but we know nothing about what sort of atmosphere or surface conditions it has.

"It's in the 'sweet spot', the Goldilocks Zone, that would allow for liquid water, but we don't know if there's actually water on it – water may never have been present. We really should be cautious.

Further, even if life exists, it is unlikely to be a thriving civilisation of advanced beings who can teach us the secrets of the universe. "The star it orbits is an M-class red dwarf," says Dr Dartnell, "so it's cooler and dimmer than our own sun. The habitable zone around it is far closer in, which means that it's tidally locked – one side always faces its star. It'll have a pretty weird, extreme climate.

"Basically, delicate creatures like animals and plants might not have had the stability required to evolve. Life, if it exists, is probably nothing more than bacterial pond scum."

Obviously that would still be amazingly exciting, and the fact that it's so close (20 light years is practically next door even by the standards of our galaxy, let alone the universe) suggests that similar planets may be everywhere. But, finally, we simply don't know how easy it is for life to come about. Prof Vogt says that the "ubiquity and propensity of life to flourish wherever it can" makes him confident that life will exist anywhere in the universe that can support it – but actually, we know nothing about how likely life is to emerge in the first place.

"The fact that we find life in almost every moist environment on Earth tells us how adaptable life is", says Dr Dartnell, "but with a single datum, you can't say anything about how likely it is to appear elsewhere." Life appeared very early in Earth's history – 300 million years after it was formed, when the crust was barely cool. But that may just have been one lucky break; we can't know until we have more data.

"That's why finding life on Mars would be so profound", says Dr Dartnell. "If we knew that life formed on two neighbouring planets independently, we could reasonably say that we could expect to find it throughout the universe." Until then, this is a fascinating find, but I suspect even Prof Vogt, when he comes back down from the giddy high of discovery, will agree that the "100 per cent" figure is a little premature.